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% fsm(Transitions, Initial, Final)
%
% fsm(-list, -nonvar, -list)
fsm([red-0-red, red-1-green, red-2-red, % a simple finite state machine example
yellow-0-red, yellow-1-green, yellow-2-red,
green-0-yellow, green-1-yellow, green-2-red],
red,
[red]).
:- object(fsm(_transitions, _initial, _final),
imports(private::assignvars)).
:- info([
version is 1.0,
author is 'Paulo Moura',
date is 2005/1/8,
comment is 'A simple implementation of finite-state machines using assignable variables and parametric objects. Adapted from a similar example by Nobukuni Kino.',
parnames is ['Transitions', 'Initial state', 'Final states']]).
:- public(recognise/1).
:- mode(recognise(+list), zero_or_more).
:- info(recognise/1,
[comment is 'Recognise a list of events.',
argnames is ['Events']]).
recognise(Events) :-
parameter(2, Initial),
::assignable(Current, Initial),
recognise(Events, Current).
recognise([], State) :-
::State => Current,
final_state(Current).
recognise([Event| Events], State) :-
::State => Current,
transition(Event, Current, Next),
(write(Current-Event-Next), nl
;
write('backtracking...'), nl, fail),
::State <= Next,
recognise(Events, State).
transition(Event, Current, Next) :-
parameter(1, Transitions),
transition(Transitions, Event, Current, Next).
transition([Current-Event-Next| _], Event, Current, Next).
transition([_| Transitions], Event, Current, Next):-
transition(Transitions, Event, Current, Next).
final_state(State) :-
parameter(3, Final),
final_state(Final, State).
final_state([State| _], State).
final_state([_| States], State) :-
final_state(States, State).
:- end_object.
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